By Sean Nicholls, Matt O'Sullivan

Updated1 December 2016 — 4:40pmfirst published at 3:58pm

Treasurer Gladys Berejiklian is refusing to release documents that would show whether or not she was advised about the real reasons for a $549 million blowout to the cost of the Sydney CBD light rail project weeks before she issued information misleading the public.

An auditor-general's report on Wednesday revealed that in October 2014 Transport for NSW had "reported" that 94 per cent of the increase was "due to incorrect estimates" in the project's business case a year earlier.

Yet Ms Berejiklian, who was transport minister at the time, issued a media release two months later stating the reason the cost had increased from $1.6 billion to $2.1 billion was due to "customer improvements to the original scope" such as longer trams and stops.

Ms Berejiklian has claimed the statement was based on what she knew at the time but refused a request to release documentation showing whether or not she was made aware of the report referred to in the auditor-general's report.

A spokesman said Ms Berejiklian had "nothing further to add at this stage".

In response to a request for the report in question, a spokesman for Transport for NSW advised Fairfax Media to lodge an application under the Government Information (Public Access) Act.

Mr Constance also refused on Thursday to reveal whether he or his ministerial colleagues received the October 2014 Transport for NSW report.

"The Auditor-General identified a number of key recommendations yesterday, which have been met," he said.

"That should give the public confidence the government has responded. We did a mea culpa yesterday. We have taken the lessons."

But Labor transport spokeswoman Jodi McKay said: "There is documentation there that either backs up what Gladys Berejiklian and Andrew Constance are saying or it completely exposes their mis-truths."